The present invention is related to a process and apparatus for producing monocrystalline ribbons or webs of suitable material. In particular, the present invention relates to a process and apparatus for drawing monocrystalline ribbons or webs from a melt comprising materials such as silicon, germanium, intermetallic compounds, sapphire, garnet, and the like.
The growth of monocrystalline ribbons of suitable materials such as silicon has been accomplished by several processes such as Dendritic Web, Stepanov, and Edge Defined Film Fed Growth (EFG). The Dendritic Web procedure entails a solidification of the silicon sheet from a melt meniscus which is bounded at the ends by thin dendrites growing downward into a supercooled melt region and bounded at the top by a ribbon-melt interface. The Stepanov method utilizes a non-wetting die in contact with the melt. The die shapes the meniscus for ribbon growth. In the EFG technique, the crystal is grown from a thin molten zone of liquid at the top surface of a capillary die. As the crystal grows, fresh liquid is supplied from a melt reservoir in a crucible via capillary rise through channels in the die. For a more detailed discussion of these methods reference is made to U.S. Pat. No. 4,075,005 to Ciszek, et al.
U.S. Pat. No. 4,000,030 to Ciszek discloses an alternative to the EFG process for growing monocrystalline materials. This process utilizes the surface tension of the melt to form a raised meniscus of the melt about a projection extending above the melt surface. The meniscus, of the melt, conforms to the cross-section of the projection to enable control of the configuration of the drawn crystal.
While the above-described techniques have been used to grow ribbons possessing adequate crystallographic structure they do suffer from numerous disadvantages. The Dendritic Web process is limited to a fixed (111) crystal orientation, and width control is difficult due to dendrite spreading. In addition, that procedure requires exacting temperature control. The other methods, described above, while in principle allowing for any crystal surface orientation, produce crystal structures which are severely impaired due to contamination of the melt by the die, and the short distance or meniscus height between the die-top and the solidification front.